1. Field of Invention
The present invention relates to an image signal input apparatus for inputting to a processing unit an image signal formed by superimposing a brightness signal on a color subcarrier wave having a generally sine waveform.
2. Description of Related Art
Conventional image signals, such as video signals, television signals and the like, are formed by superimposing a brightness signal on a color subcarrier wave that indicates the color of each pixel; the brightness signal varies in strength with the brightness of each pixel. These video signal may be produced by a digital camera. A conventional apparatus for inputting these image signals to a processing unit, such as a computer or the like, converts the image signal into digital data by sampling the image signal at a frequency that is 4 or 8 times the frequency (fsc) of the color subcarrier wave (i.e., a frequency of 4 fsc or 8 fsc). The conventional apparatus then inputs all the data obtained by sampling to the computer or the like.
The data obtained by sampling at a frequency of 4 fsc or 8 fsc can easily be demodulated into an original image signal. Therefore, a processing unit, such as a computer, extracts a brightness signal from the demodulated image signal, performs a preview operation of displaying a corresponding image in a monochrome view and, when a capture key is operated during the preview operation, performs a capture operation of storing in a memory the image signal available at the time of the operation of capture key.
However, the quantity of data obtained by sampling at a frequency of 4 fsc or 8 fsc is large. Therefore, a long processing time is required to input the data to the computer, perform the preview operation based on the data, and optionally perform the capture operation. Therefore, the progression of the preview operation is slower than the progression of the image signal unless the number of frames displayed during preview is considerably reduced. In addition, the displayed monochrome images are displayed after a significant delay time because of the unequal progression of the preview operation and the image signal. Consequently, it is very difficult, if not impossible to precisely input an instruction to perform image signal capture at a desired time by operating a capture key. Further, during processing, operations other than the preview and capture operations are performed on the image signal. Therefore, the problem of a long processing time is compounded. Thus, a reduction in such processing time is needed.